|Publication number||US6982126 B2|
|Application number||US 10/707,197|
|Publication date||Jan 3, 2006|
|Filing date||Nov 26, 2003|
|Priority date||Nov 26, 2003|
|Also published as||EP1536039A1, US20050112412|
|Publication number||10707197, 707197, US 6982126 B2, US 6982126B2, US-B2-6982126, US6982126 B2, US6982126B2|
|Inventors||Ramgopal Darolia, Irene Spitsberg, Brett Allen Rohrer Boutwell, Mark Daniel Gorman, Curtis Alan Johnson, Venkat Subramaniam Venkataramani|
|Original Assignee||General Electric Company|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (32), Non-Patent Citations (2), Referenced by (40), Classifications (36), Legal Events (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention was made with Government support under Agreement No. N00421-00-3-0443 awarded by Naval Air Warfare Center Aircraft Division. The Government has certain rights in the invention.
1. Field of the Invention
This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating (TBC) having a columnar microstructure, an interior region formed of a ceramic material with lower thermal conductivity than yttria-stabilized zirconia (YSZ), and an outer surface region that is more erosion-resistant than the interior region of the TBC.
2. Description of the Related Art
Components within the hot gas path of gas turbine engines are often protected by a thermal barrier coating (TBC). TBC's are typically formed of ceramic materials deposited by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Spraying techniques deposit TBC material in the form of molten splats, resulting in a TBC characterized by a degree of inhomogeneity and porosity. TBC's employed in the highest temperature regions of gas turbine engines are most often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., laser melting, etc.).
Various ceramic materials have been proposed as TBC's, the most widely used being zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO), or ceria (CeO2) to yield a tetragonal microstructure that resists phase changes. Other stabilizers have been proposed for zirconia, including hafnia (HfO2) (U.S. Pat. No. 5,643,474 to Sangeeta), gadolinia (Gd2O3) (U.S. Pat. Nos. 6,177,200 and 6,284,323 to Maloney), and dysprosia (Dy2O3), erbia (Er2O3), neodymia (Nd2O3), samarium oxide (Sm2O3), and ytterbia (Yb2O3) (copending U.S. patent application Ser. No. 10/064,939). Yttria-stabilized zirconia (YSZ) has been the most widely used TBC material due at least in part to its high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and PVD techniques.
TBC materials that have lower thermal conductivities than YSZ offer a variety of advantages, including the ability to operate a gas turbine engine at higher temperatures, increased part durability, reduced parasitic cooling losses, and reduced part weight if a thinner TBC can be used. Commonly-assigned U.S. Pat. No. 6,586,115 to Rigney et al. discloses a YSZ TBC alloyed to contain an additional oxide that lowers the thermal conductivity of the base YSZ composition. These additional oxides include alkaline-earth metal oxides (magnesia, calcia (CaO), strontia (SrO) and barium oxide (BaO)), rare-earth metal oxides (ceria, gadolinia, neodymia, dysprosia and lanthana (La2O3)), and/or such metal oxides as nickel oxide (NiO), ferric oxide (Fe2O3), cobaltous oxide (CoO), and scandium oxide (Sc2O3). According to Rigney et al., when present in sufficient amounts these oxides are able to significantly reduce the thermal conductivity of YSZ by increasing crystallographic defects and/or lattice strains. Other ternary YSZ coating systems that have been proposed include YSZ+hafnia (commonly-assigned U.S. Pat. No. 6,352,788 to Bruce) and YSZ+niobia (Nb2O3) or titania (TiO2) (commonly-assigned co-pending U.S. patent application Ser. No. 10/063,810 to Bruce et al.).
In addition to low thermal conductivities, TBC's on gas turbine engine components are required to withstand damage from impact by hard particles of varying sizes that are generated upstream in the engine or enter the high velocity gas stream through the air intake of a gas turbine engine. The result of impingement can be erosive wear (generally from smaller particles) or impact spallation from larger particles. Many of the oxides noted by Rigney et al. as able to reduce the thermal conductivity of YSZ have the disadvantage of also reducing erosion resistance. Above-noted U.S. Pat. No. 6,352,788 to Bruce teaches that YSZ containing about one up to less than six weight percent yttria in combination with magnesia and/or hafnia exhibits improved impact resistance. In addition, commonly-assigned U.S. patent application Ser. No. 10/063,962 to Bruce shows that small additions of lanthana, neodymia and/or tantala to zirconia partially stabilized by about four weight percent yttria (4% YSZ) can improve the impact and erosion resistance of 4% YSZ.
It would be desirable if improved TBC's were available that exhibited both lower thermal conductivities and improved erosion resistance.
The present invention generally provides a thermal barrier coating (TBC) for a component intended for use in a hostile thermal environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The TBC has an interior region and an outer surface region on and contacting the interior region. Both regions are formed of a ceramic material, with the ceramic material of at least the interior region having a lower thermal conductivity than zirconia partially stabilized by about seven weight percent yttria, e.g., zirconia (ZrO2) containing one or more of ytterbia (Yb2O3), neodymia (Nd2O3), lanthana (La2O3), hafnia (HfO2), tantala (Ta2O3), gadolinia (Gd2O3), erbia (Er2O3), dysprosia (Dy2O3), and/or yttria (YO3; e.g., more than 7 weight percent yttria if yttria is the only additive oxide). The interior region constitutes more than half of the thickness of the TBC, and the outer surface region constitutes less than half of the thickness of the TBC. The TBC has a columnar microstructure whereby the interior region and the outer surface region comprise columns of their ceramic materials. According to the invention, the outer surface region is more erosion and impact resistant than the interior region at least in part as a result of the columns thereof being more closely spaced than the columns of the interior region. In addition, the outer surface region may be formed to contain less porosity than the interior region.
As a result of the low thermal conductivity and relative thickness of the interior layer, the TBC preferably has a lower thermal conductivity than an equivalent columnar YSZ TBC while also exhibiting enhanced erosion and impact resistance as a result of the denser columnar construction of the outer surface region, which serves to protect the less erosion-resistant interior region. Accordingly, the TBC of this invention allows for the use of thinner coatings and/or lower cooling airflow for air-cooled components. Both regions of the TBC can be readily deposited by PVD techniques to have the more strain-resistant columnar grain structure noted above, with the closer column spacing of the outer surface region being achieved through compositional or processing modifications.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
While the present invention is applicable to a variety of components subjected to high temperatures, such as the high and low pressure turbine nozzles and blades, shrouds, combustor liners and augmentor hardware of gas turbine engines, the invention will be discussed in reference to a high pressure turbine blade 10 shown in
The TBC system 20 is represented in
According to the invention, the TBC 26 has two regions: an interior region 32 and an outer surface region 34 (i.e., forming the outer surface of the TBC 26), as schematically represented in
To promote the thermal insulative properties of the TBC 26, at least the interior region 32 is preferably formed of a ceramic material having a lower thermal conductivity than the more conventional zirconia partially stabilized by about seven weight percent yttria (7% YSZ; thermal conductivity of about 1.6 to 2.0 W/mK at 900° C.). Preferred ceramic materials include zirconia containing one or more of ytterbia, neodymia, lanthana, and hafnia. When deposited by EBPVD to have a columnar grain structure, these ceramic materials have thermal conductivities of less than 1.4 W/mK, typically about 0.8 to 1.4 W/mK, as a result of the additive oxides contributing to one or more of three characteristics capable of reducing the thermal conductivity of a zirconia-based material: an atomic weight greater than that of zirconia, more preferably greater than that of yttria, to introduce phonon scattering sites and thereby reduces phonon free path to a greater extent than yttria; an atomic radius different than that of zirconia, more preferably greater than that of yttria, again to introduce phonon scattering; and a valance different than that of zirconia to introduce oxygen defects at a controlled level, providing another phonon scattering mechanism. Yttria can also be used as an additive oxide to zirconia for purposes of this invention, though preferably in amounts greater than 7 weight percent (for example, 20 weight percent) if it is the only additive oxide since the thermal conductivity of YSZ decreases with increasing yttria content. Effective amounts of ytterbia, yttria, neodymia, lanthana, and hafnia are believed to be, by weight, about 1% to about 65%, with additive oxide contents above about 50 weight percent yielding lowconductivity zirconia-based compounds such as pyrochlores, e.g., lanthanum zirconate. Tantala, gadolinia, erbia, and dysprosia, used alone, in any combination with each other, or in any combination with ytterbia, yttria, neodymia, lanthana, and/or hafnia, are also effective when present in an amount of about 1 to about 65 weight percent. Other oxide additives can also be used if they meet one or more of the above conditions, such as alkaline-earth metal oxides (e.g., MgO, CaO, SrO, BaO), other rare-earth metal oxides (e.g., CeO2, Sm2O3), Nb2O5, NiO, Fe2O3, CoO, Sc2O3. etc. Preferably, yttria and ytterbia, neodymia, lanthana, hafnia, tantala, gadolinia, erbia, and/or dysprosia are present in a combined amount of about 7 to about 14 weight percent, with the balance essentially zirconia. Depending on the composition of the interior layer 32, an underlayer (not shown) formed of YSZ (e.g., 7% YSZ) and having a thickness of about 0.5 to about 2 mils (about 10 to about 50 micrometers) may be desirable to promote the adhesion of the TBC 26 to the substrate 22, thereby improving spallation resistance.
Various process and composition-related approaches can be used to obtain the different microstructures within the interior and outer surface regions 32 and 34 of the TBC 26, as will be discussed below. The compositions of the regions 32 and 34 may be identical (resulting in a constant composition throughout the TBC 26), have the same base composition but modified with certain additions, or have different base compositions. If the regions 32 and 34 have the same composition, processing modifications must be made to result in the denser microstructure desired for the outer surface region 34. If the regions 32 and 34 have the same base composition, minor chemistry modifications can be made to the outer surface region 34 to enhance surface diffusion processes and promote flatness of the crystallization front, causing a majority of the inter-columnar gaps to decrease during the deposition process. Examples of such chemistry modifications include additions of nickel, titanium, chromium, and/or their oxides to enhance sintering processes in zirconia during deposition of the outer surface region 34. Finally, if the interior and outer surface regions 32 and 34 are formed of different base compositions, the outer surface region 34 can be formed of a more erosion and impact-resistant ceramic material, such as YSZ and particularly in the range of about 4% YSZ to about 8% YSZ, whose microstructure can be densified by making appropriate processing modifications during deposition of the outer surface region 34.
In an investigation leading to this invention, YSZ TBC's having a nominal yttria content of about seven weight percent were deposited by EBPVD to have thicknesses of about 150 micrometers. Each of the TBC's were deposited on pin specimens formed of René N5 (nominal composition of, by weight, about 7.5% Co, 7.0% Cr, 6.5% Ta, 6.2% Al, 5.0% W, 3.0% Re, 1.5% Mo, 0.15% Hf, 0.05% C, 0.004% B, 0.01% Y, the balance nickel and incidental impurities), on which a platinum aluminide (PtAl) bond coat had been deposited. The microstructures of the TBC's differed from each other primarily as a result of modifications to the EBPVD process (e.g., deposition temperature, deposition pressure, rotation). The compositions of the TBC's were varied by small variations in the yttria content (about 4 to about 7 weight percent) and/or additions of small amounts (up to 5 weight percent) of ytterbia, lanthana, hafnia, or tantala. Twelve TBC's were deposited and analyzed for microstructure before and after undergoing impact and erosion testing. On the basis of this analysis, the TBC's were numerically assigned one of five types based on the columnarity and featheriness of the grain columns and the spacing between adjacent grain columns. The coating microstructures are summarized below in Table I.
ture Type (#-
5.3 La2O3 +
3.4 Yb2O3 +
1.8 to 2.4
0.1 to 3.9
5.0 Yb2O3 +
Scanned images of coatings 1 and 2 are shown in
Impact and erosion tests were conducted on the specimens at high temperatures (about 1220° C.) and high gas velocities (about Mach 0.5) while subjected to a particulate material injected into the gas stream. An alumina powder with an average particle size of about 560 micrometers was used to evaluate impact resistance, while an alumina powder with an average particle size of about 50 micrometers was used to evaluate erosion resistance. The specimens were rotated at a rate of about 500 rpm while subjected to the high velocity powder. Both erosion and impact resistance were measured in terms of the number of grams of erodent required to wear away one mil (about 25 micrometers) of TBC. The results are reported in Table II below.
Numerical analysis associating erosion and impact resistance with microstructure type showed that, when all coating characteristics were considered, dense coating microstructures with no gaps between columns (e.g., the closed columns of coating 1 shown in
In a second investigation, the extent to which EBPVD deposition temperature can be used to control coating microstructure was assessed with YSZ TBC's having a nominal yttria content of about seven weight percent. The coatings were again deposited by EBPVD to have thicknesses of about 150 micrometers. Each coating was deposited on a pin specimen formed of René N5, on which a PtAl bond coat had been deposited. The microstructures of the outer surface regions of the TBC's were modified relative to the interior regions of the coatings by increasing the power of the electron beam while depositing the final twenty-five micrometers of the coatings. The electron beam power was held at about 2.5 kW during deposition of the interior coating regions, after which power was periodically increased in either five-second pulses to about 18 to 20 kW or ten-second impulses to about 15 to 16 kW (and therefore lower power but higher energy).
The hardness of each coating was measured using a Vickers pyramid with a fifty gram load. Hardness readings indicated that the interior regions of all coatings had hardness readings on the order of about 3 to 5.2 GPa. Those coatings with dense columnar microstructures represented by
In addition to process changes, selective densification of the outer surface region 34 can be achieved by adding small amounts of nickel, titanium, chromium, and/or their oxides, which are known to enhance sintering processes in zirconia. Suitable amounts of these additives are believed to be on the order of about 1 to about 2 weight percent of the TBC material.
The above investigations were conducted with YSZ TBC to characterize the types of coating microstructure associated with improved erosion and impact resistance. Lower thermal conductivities sought by this invention can be achieved by substituting a low thermal conductivity ceramic material for YSZ within the interior region of the TBC. From the results obtained with YSZ coatings, it was concluded that improvements in erosion and impact resistance can be achieved with a denser columnar microstructure at the outer surface region of the TBC, regardless of the ceramic material used, including those ceramic materials known to have lower thermal conductivities but also lower erosion/impact resistance than YSZ. With this approach, the lower thermal conductivities and improved erosion/impact resistance sought by this invention can be simultaneously achieved by using a low thermal conductivity ceramic material throughout the TBC.
While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.
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|U.S. Classification||428/701, 428/141, 428/148, 428/697, 428/632, 428/699, 428/702, 428/633, 416/241.00B|
|International Classification||F01D5/28, C23C28/00, B32B9/00, C23C14/08, F03B3/12|
|Cooperative Classification||Y10T428/24413, Y10T428/12611, Y10T428/24355, Y10T428/12618, F05D2230/90, F05D2300/611, C23C28/321, C23C14/08, C23C28/3215, Y02T50/671, C23C28/3455, C23C14/083, F01D5/288, C23C28/345, Y02T50/67|
|European Classification||C23C28/3455, C23C28/321, C23C28/345, C23C28/3215, F01D5/28F, C23C14/08H, C23C14/08|
|Dec 24, 2003||AS||Assignment|
Owner name: GENERAL ELECTRIC COMPANY, NEW YORK
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:DAROLIA, RAMGOPAL NMN;SPITSBERG, IRENE NMN;BOURTWELL, BRETT ALLEN ROHRER;AND OTHERS;REEL/FRAME:014217/0253;SIGNING DATES FROM 20031208 TO 20031216
|Mar 27, 2004||AS||Assignment|
Owner name: GENERAL ELECTRIC COMPANY, NEW YORK
Free format text: CORRECTED ASSIGNMENT RECORDATION COVER SHEET;ASSIGNORS:DAROLIA, RAMGOPAL;SPITSBERG, IRENE;BOUTWELL,BRETT ALLEN ROHRER;AND OTHERS;REEL/FRAME:015162/0833
Effective date: 20031224
|Jul 6, 2009||FPAY||Fee payment|
Year of fee payment: 4
|Aug 16, 2013||REMI||Maintenance fee reminder mailed|
|Jan 3, 2014||LAPS||Lapse for failure to pay maintenance fees|
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